Prior work has demonstrated that antisocial behavior in one sibling is significantly correlated with the emergence of similar behavior across time in other siblings (e.g., Farrington, Barnes, & Lambert, 1996; Wasserman, et al., 1996). Although sibling interventions have been studied only rarely (Johnston & Freeman, in press), a number of intervention studies have reported findings in which treatment of an older sibling with antisocial behavior problems has resulted in reductions in antisocial behaviors in younger siblings as compared to control group younger siblings. Bank et al. (1997) have demonstrated that high levels of sibling Conflict during middle childhood are predictive of poor adjustment outcomes--including antisocial behavior -- during adolescence. The Bank et al. findings were statistically reliable in the context of parenting variables, suggesting that sibling intervention components as a companion to parent training interventions should be evaluated. In the present study, I proposed that 150 families with a 6- to 8-year-old child with conduct problems be recruited from Oregon Health Plan participants. Each of these children will have a younger sibling at least 4 years of age. Families will be randomly assigned to one of three groups--Parent Training (N=50), Sibling and Parent Training (N=50), and a Control (N=50--who will receive intervention as usual from Lane County service providers. There are three central hypotheses. First, at intervention completion and 1-year follow-up, sibling conflicts reported by parents and coded during home observations will be significantly reduced in those families receiving the siblings' intervention component (SPT) as compared to the Parent Training (PT) and Control (CT) families; second, for the SPT older siblings at termination and follow-up and for their younger siblings at follow-up in the school setting, peer skills are expected to be significantly better than in the PT or CT groups. Third, at intervention completion and 1-year follow-up, older siblings in both intervention groups will show diminished levels of conduct problems and improved levels of academic skills as compared to control older siblings; among younger siblings, however, at follow-up in the school setting, SPT, but not PT, will exhibit significantly fewer conduct problems and better academic skills than CT younger siblings. Multiple-agent, multiple-method data--including observation of home, school, and laboratory interactions--will be collected at baseline, intervention completion, and follow-up for key constructs for Parent Discipline, Parent Supervision, Problem Solving, Sibling Negative Interaction, Antisocial Behavior, Academic Skills, and Peer Relations. MANOVA and structural equation modeling will be used to test the key hypotheses.